Gate all around semiconductor structure with diffusion break

ABSTRACT

The current disclosure describes techniques for forming semiconductor structures having multiple semiconductor strips configured as channel portions. In the semiconductor structures, diffusion break structures are formed after the gate structures are formed so that the structural integrity of the semiconductor strips adjacent to the diffusion break structures will not be compromised by a subsequent gate formation process. The diffusion break extends downward from an upper surface until all the semiconductor strips of the adjacent channel portions are truncated by the diffusion break.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as GAAstructures. Non-Si based low-dimensional materials are promisingcandidates to provide superior electrostatics (e.g., for short-channeleffect) and higher performance (e.g., less surface scattering). Carbonnanotubes (CNTs) are considered one such promising candidate due totheir high carrier mobility and substantially one dimensional structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A and 1B illustrate an example GAA structure with diffusion breakstructures in accordance with the present disclosure;

FIGS. 2A and 2B illustrate another example GAA structure with diffusionbreak structures in accordance with the present disclosure;

FIG. 3 is an example fabrication process in accordance with the presentdisclosure; and

FIGS. 4A-23 illustrate a wafer in various stages of fabrication underthe example fabrication process of FIG. 3.

DETAILED DESCRIPTION

Semiconductor structures, such as CMOS devices, continue to be scaled tosmaller sizes to meet advanced performance targets. Due to the extrahigh density and extra small device dimensions, after metal gates areformed in a semiconductor structure, some metal gate structures orsacrificial gate structures may need to be removed by etching to formgate line-end regions, for various reasons. For example, gate line-endregions are used to achieve end cap space or to isolate separate logicactive areas through a diffusion break.

The current disclosure describes techniques for forming semiconductorstructures having multiple vertically arranged semiconductor stripsconfigured as channel portions. In the semiconductor structures,diffusion break structures are formed subsequent to the gate structuresso that the structural integrity of the semiconductor strips adjacent tothe diffusion break structures will not be compromised by a subsequentgate formation process. The diffusion break structures each extendsdownward from an upper surface, e.g., about a same level as uppersurfaces of the gate structures, until all the semiconductor strips ofthe adjacent channel portions are truncated by the diffusion breakstructure. The semiconductor strips of the adjacent channel region referto the semiconductor strips used or to be used as channel portions forthe devices adjacent to the diffusion break. In a case that an adjacentdevice does not use all the vertically arranged semiconductor strips aschannel strips, the diffusion break structure may truncate only thesemiconductor strips that are used as channel strips. It should beappreciated that devices in an integrated circuit or formed on a samesemiconductor die or wafer may include different numbers ofsemiconductor strips in the channel portions thereof. So it is possiblethat the diffusion break structures may have different depths and maytruncate different number of semiconductor strips.

In an embodiment, a diffusion break structure truncates all thesemiconductor strips vertically stacked over the substrate, but does notextend into the substrate. That is, the diffusion break structure isformed over the substrate. The diffusion break structure may be formedbetween two immediately adjacent gate structures, e.g., referred to as“double diffusion break,” or may be formed by removing at leastpartially a gate structure, e.g., a gate electrode of a gate structure,and replacing the removed gate structure with a dielectric material,which is referred to as “single diffusion break.” For each of the doublediffusion break structure or the single diffusion break structure, thedepth of the diffusion break is configured to be sufficiently large totruncate the semiconductor strips but does not extend into the substrateunder the semiconductor strips.

The diffusion break structures are formed after the gate structures,sacrificial gate structures or replacement gate structures, are madeover the semiconductor strips. More specifically, the diffusion breakstructures separate the semiconductor strips. As such, the diffusionbreak structure contacts the edge surfaces of the truncatedsemiconductor strips. The semiconductor strips are truncated by etchingbefore the diffusion break structure is formed. So the edge surfaces ofthe truncated semiconductor strips include facet shapes that are createdby the etching. Such facet shapes of the edge surfaces may be differentthan the facet shapes of the semiconductors edge surfaces created by anepitaxial process.

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific embodiments or examples of components andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. For example, dimensions of elements are not limited to thedisclosed range or values, but may depend upon process conditions and/ordesired properties of the device. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact. Variousfeatures may be arbitrarily drawn in different scales for simplicity andclarity. In the accompanied drawings, some layers/features may beomitted for simplification.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The device may be otherwise oriented (rotated 90 degrees orat other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“made of” may mean either “comprising” or “consisting of” Further, inthe following fabrication process, there may be one or more additionaloperations in/between the described operations, and the order ofoperations may be changed. In the present disclosure, a phrase “one ofA, B and C” means “A, B and/or C” (A, B, C, A and B, A and C, B and C,or A, B and C), and does not mean one element from A, one element from Band one element from C, unless otherwise described.

In some embodiments, semiconductor devices include a novel structure offield-effect transistors including stacked, gate-all-around (GAA)semiconductor channel strips of nanowire, nanosheet, or carbon nanotubes(CNTs). The semiconductor devices include an array of aligned channelstrips with a gate dielectric layer wrapping therearound and a gateelectrode layer. The gate-all-around field effect transistors (GAA FETs)can be applied to logic circuits in advanced technology node. In thedescription herein, a GAA device having semiconductor channel strips ofCNTs are used as an illustrative example to describe the currenttechniques. It should be appreciated that similar techniques can also beapplied to other type of semiconductor channel strips, which areincluded in the disclosure.

FIGS. 1A-1B show an example structure 100 having a double diffusionbreak structure. FIG. 1A is a top plane view from cross-section line A-A(FIG. 1B). FIG. 1B is a side cross-sectional view from cross-sectionalline B-B (FIG. 1A). Referring to FIGS. 1A and 1B together, the structure100 includes a substrate 110 and a plurality of (shown as four) stacks112, 114, 116, 118 of semiconductor strips over the substrate 110. Eachof the plurality of stacks 112, 114, 116, 118, shown as the stack 114,includes a plurality of semiconductor layers 114(1), 114(2), 114(3),114(4) arranged vertically with respect to one another. Eachsemiconductor layer 114(1), 114(2), 114(3), 114(4) may also include oneor more semiconductor strips, e.g., nanowires, nanosheets or carbonnanotubes, arranged laterally as a group, which is also included in thedisclosure. In the description herein, without losing generality, thenumerals 112, 114, 116, 118 are also used to generally refer to asemiconductor layer or a semiconductor strip in a respective stack. Asemiconductor nanowire generally refers to a strip-shaped semiconductorlayer that has a substantially circular cross-sectional shape with adiameter ranging between about 2 nm and about 15 nm. A semiconductornanosheet generally refers to a strip-shaped semiconductor layer thathas a substantially rectangular cross-sectional shape with a heightranging between about 2 nm and about 10 nm and a width ranging betweenabout 4 nm to about 50 nm. In the description herein, for descriptivepurposes, the term “semiconductor strip” is used generally to refer to adiscrete semiconductor layer that is strip-shaped and includes variouscross-sectional shapes including, but not limited to, nanowire ornanosheet.

The semiconductor strips 112, 114, 116, 118 may be Si, Ge, SiGe, GaN,GaAs, InN, InAs, CNT or other suitable semiconductor materials.

Two circuit regions 120, 130 are formed over the stacks 112, 114, 116,118 of semiconductor strips. The two circuit regions 120, 130 areseparated by a diffusion break structure 140. Specifically, thediffusion break structure 140 truncates the semiconductor strips 112,114, 116, 118 between the two circuit regions 120, 130 such thatcurrents do not flow between the two regions 120, 130 through thesemiconductor strips 112, 114, 116, 118. More specifically, thediffusion break structure 140 separates the semiconductor strips 112,114, 116, 118 into two vertical arrays in the circuit regions 120, 130,respectively. The semiconductor strips 112, 114, 116, 118 in the twoarrays are in lateral alignment with one another. For each of the twocircuit regions 120, 130, the currents are not designed to flow or leakthrough the substrate 110. As such, the diffusion break structure 140extends from an upper surface 142 to a lower surface 144 that is higherthan or substantially at a same level as an upper surface of thesubstrate 110. That is, the diffusion break structure 140 is formed overthe substrate 110 and is not embedded within the substrate 110. Thelower surface 144 is lower than the lowest semiconductor strip, e.g.,114(4), in the respective stack 112, 114, 116, 118 such that thediffusion break structure 140 separates all the semiconductor strips112, 114, 116, 118. In the description of the relative positions amongstructures or layers, the relative terms of “higher” or “lower” “upper”or “bottom” are used in respect to the substrate 110, which is definedas lower than all the structures formed thereover.

Each of the circuit regions 120, 130 may include one or more gatestructures 150 and two or more source/drain structures 160 adjacent tothe gate structures 150. The gate structure 150 includes gate electrode152 and a gate dielectric layer (not specifically shown in FIGS. 1A, 1Bfor simplicity purposes). Optionally, a spacer 162 of a dielectricmaterial, e.g., silicon oxide or silicon nitride, insulates the gatestructure 150 from the adjacent source/drain structure 160. Forsimplicity purposes, FIG. 1A does not show the spacer 162. In anembodiment, the lower surface 144 of the diffusion break structure 140is substantially at a same level as a bottom surface of the gatestructure 150, which wraps around all the semiconductor strips 112, 114,116, 118.

As shown in the double diffusion break scenario of FIG. 1A, 1B, thediffusion break structure 140 is positioned more proximate to the mostadjacent gate structure 150 than to the most adjacent source/drainstructure 160. In an embodiment, the diffusion break structure 140 isformed in a space between two immediately adjacent gate structures 150.In an embodiment, a width W1 of the diffusion break structure 140 issubstantially equal to a width W2 of a source/drain structure 160.

In an embodiment, the diffusion break structure 140 interfaces withtermination edges 170 of the semiconductor strips 112, 114, 116, 118.Because the diffusion break structure 140 is formed after the gatestructure 150 and/or the spacers 162 are formed, the termination edgesurface 170 of the semiconductor strips 112, 114, 116, 118 may besubstantially plumb with the adjacent spacer 162.

FIGS. 2A, 2B show an example structure 200 of a single diffusion breakstructure. The structure 200 is very similar to the structure 100 exceptfor the single diffusion break structure 240. As shown in FIGS. 2A, 2B,the single diffusion break structure 240 is more proximate to the mostadjacent source/drain structure 160 than to the most adjacent gatestructure 150. A width W3 of the single diffusion break is substantiallyequal to a width W4 of the gate structure 160. In some circuit designs,the gate structures 150 may include different width, e.g., long gateversus short gate. The width W3 of the diffusion break structure 240 issubstantially similar to the width of one of the long gate or the shortgate, usually the short gate.

Referring to FIGS. 1A, 1B, 2A, 2B together, the gate structures 150 wraparound sidewall surfaces of the semiconductor strips 112, 114, 116, 118to maximize the gate control of the charge carrier flow through thesemiconductor strips 112, 114, 116, 118. The charge carriers areprevented from flowing through the substrate 110 via one or more of gatecontrol, doping, separation or insulation. For example, the lowestsemiconductor strip 114(4) is separated from the substrate 110 by a gap172. For example, the substrate 110 may be doped to have P-N junctionwith the source/drain structure 160 to prevent charge carriers fromflowing between the source/drain structure 160 and the substrate 110.

The substrate 110 may include a silicon substrate in crystallinestructure and/or other elementary semiconductors like germanium.Alternatively or additionally, the substrate 110 may include a compoundsemiconductor such as silicon carbide, gallium arsenide, indiumarsenide, and/or indium phosphide. Further, the substrate 110 may alsoinclude a silicon-on-insulator (SOI) structure. Substrate 110 mayinclude an epitaxial layer and/or may be strained for performanceenhancement. The substrate 110 may also include various dopingconfigurations depending on design requirements such as P-type substrateand/or N-type substrate and various doped regions such as P-wells and/orN-wells.

The gate structures 150 are replacement gate structures. The followingdescription lists examples of materials for the gate structure 150including the gate electrode 152 and the gate dielectric (notspecifically shown in FIGS. 1A, 1B, 3A, 2B for simplicity), which arenon-limiting. The gate electrode 152 includes a conductive material,e.g., a metal or a metal compound. Suitable metal materials for the gateelectrode 152 include ruthenium, palladium, platinum, tungsten, cobalt,nickel, hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta),aluminum (Al), aluminides and/or conductive metal carbides (e.g.,hafnium carbide, zirconium carbide, titanium carbide, and aluminumcarbide), and other suitable conductive materials. In some examples, thegate electrode 152 includes a work function adjustment layer tuned tohave a proper work function for enhanced performance of the field effecttransistor devices. For example, suitable N-type work function metalsinclude Ta, TiAl, TiAlN, TaCN, other N-type work function metal, or acombination thereof, and suitable P-type work function metal materialsinclude TiN, TaN, other P-type work function metal, or combinationthereof. In some examples, a conductive layer, such as an aluminumlayer, a copper layer, a cobalt layer or a tungsten layer is formed overthe work function adjustment layer such that the gate electrode 152includes a work function layer disposed over the gate dielectric and aconductive layer disposed over the work function layer. In an example,the gate electrode 152 has a thickness ranging from about 5 nm to about40 nm depending on design requirements.

In example embodiments, the gate dielectric layer includes aninterfacial silicon oxide layer, e.g., thermal or chemical oxide havinga thickness ranging from about 5 to about 10 angstrom (Å). In exampleembodiments, the gate dielectric layer further includes a highdielectric constant (high-K) dielectric material selected from one ormore of hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO), hafniumsilicon oxynitride (HfSiON), hafnium tantalum oxide (HMO), hafniumtitanium oxide (HMO), hafnium zirconium oxide (HfZrO), combinationsthereof, and/or other suitable materials. A high K dielectric material,in some applications, includes a dielectric constant (K) value largerthan 6. Depending on design requirements, a dielectric material of adielectric contact (K) value of 7 or higher is used. The high-Kdielectric layer may be formed by atomic layer deposition (ALD) or othersuitable technique. In accordance with embodiments described herein, thehigh-K dielectric layer of the gate dielectric layer includes athickness ranging from about 10 to about 30 angstrom (Å) or othersuitable thickness.

The spacer 162 is formed of a low K dielectric material such as siliconoxynitride (SiO_(x)N_(y)), silicon nitride (Si₃N₄), silicon monoxide(SiO), silicon oxynitrocarbide (SiONC), silicon oxycarbide (SiOC),silicon carbide (SiC), hafnium oxide (HfO₂) vacuum and other dielectricsor other suitable materials. The spacer 162 may be formed throughchemical vapor deposition (CVD), high density plasma CVD, spin-on,sputtering, or other suitable approaches.

The material of the source/drain structure 160 may be selected based onthe materials of the semiconductor strips 112, 114, 116, 118 and thedevice designs. For example, for N-type devices of silicon strips 112,114, 116, 118, the source/drain structure 160 may include siliconcarbide (SiC), silicon carbon phosphide (SiCP), silicon phosphide (SiP)or other suitable semiconductor materials. For P-type devices of siliconor silicon germanium strips 112, 114, 116, 118, the source/drainstructure 160 may include silicon germanium (SiGe) or silicon-germanium-boron (SiGeB) or other suitable semiconductor materials. Thesource/drain structure 160 may be doped in various approaches withvarious dopants/impurities, like arsenic, phosphorous, boron, gallium,indium, antimony, oxygen, nitrogen, or various combinations thereof.

In an embodiment, the semiconductor strips 112, 114, 116, 118 are notdoped, e.g., intrinsic, to facilitate charge carrier flow under propergate control by the gate structures 150.

FIG. 3 shows an example process 300. FIGS. 4A-23 show a wafer 400 invarious stages of fabrication under the example process of FIG. 3. Ateach stage, one or more of three views of the wafer 400 are shown, i.e.,the perspective view referenced with letter “A,” a sectional view fromcutting line B-B, referenced with letter “B” and also referred to as “B”plane (X-Z plane), and a sectional view from cutting line C-C,referenced with letter “C” and also referred to as “C” plane. At some ofthe stages, only one view of the wafer 400 is shown and the referenceletter of the respective view will be omitted for simplicity.

In the FIGS. 3 and 4A-23, a sequential fabrication process of a GAA FETusing carbon nanotubes in accordance with embodiments of the presentdisclosure are used as an illustrative example for descriptive purposes.It is understood that additional operations can be provided before,during, and after processes shown by FIGS. 4A-23, and some of theoperations described below can be replaced or eliminated, for additionalembodiments of the method. The order of the operations/processes may beinterchangeable. The fabrication process may be similarly used to makeGAA FET of silicon nanowire channels, silicon nanosheet channels,semiconductor strips of other semiconductor materials, with appreciablevariations, which are all included in the disclosure. Further, thedisclosed techniques may also be used for other type of FET devices,like complementary FET devices where semiconductor channels strips ofnFET and pFET are stacked on top of one another and are adjacent to asame gate structure, e.g., common gate.

Referring to FIG. 3 and FIGS. 4A-4C, in example operation 305, a wafer400 is received. The wafer 400 includes a substrate 410, an insulationlayer 420 over the substrate 410, a plurality of carbon nanotube (CNT)layers 430 are arranged on buffer layers 440 in an alternating manner.Specifically, every two immediately adjacent CNT layer 430 isvertically, e.g., in the z-axis, separated by a buffer layer 440.

In some embodiments, the substrate 410 is made of a suitable elementalsemiconductor, such as silicon, diamond or germanium; a suitable alloyor compound semiconductor, such as Group-IV compound semiconductors(e.g., silicon germanium (SiGe), silicon carbide (SiC), silicongermanium carbide (SiGeC, GeSn, SiSn, SiGeSn), Group III-V compoundsemiconductors (e.g., gallium arsenide, indium gallium arsenide(InGaAs), indium arsenide, indium phosphide, indium antimonide, galliumarsenic phosphide, or gallium indium phosphide), or the like. Aninsulating material, such as a glass, may also be used as the substrate.

The insulation layer 420 is made of one or more layers of silicon oxide,silicon nitride, SiON, SiOC, SiOCN and SiCN, or other suitabledielectric material.

In some embodiment, the lowest CNT layer 430 is positioned directly overthe insulation layer 420. In some other embodiment, a lowest bufferlayer 440, also called as a “bottom buffer layer,” is formed between theinsulation layer 420 and the lowest CNT layer 430. The bottom bufferlayer 440 includes a polycrystalline or amorphous material of one ormore of Si, Ge or SiGe. The bottom buffer layer 440 can be formed bysuitable film formation methods, such as thermal oxidation, chemicalvapor deposition (CVD), physical vapor deposition (PVD) or atomic layerdeposition (ALD). In certain embodiments, silicon oxide (e.g., SiO₂) isused as the 440.

In the description herein, the lowest CNT layer 430 directly seating onthe insulation layer 420 is used as an illustrative example fordescriptive purposes. Further, the wafer 400 includes seven CNT layers430 and seven buffer layers 440 arranged in the alternating manner, forillustrative purposes only.

At the lowest CNT layer 430, one or more carbon nanotubes (CNTs) 450 arearranged over the insulation layer 420. The CNTs are arranged on theinsulation layer 420 substantially in alignment with one another in asame orientation, e.g., the x-axis orientation as illustratively shown.The deviation from the alignment orientation of the CNTs 450, here thex-axis orientation, is about ±10 degrees in some embodiments, and isabout ±5 degrees in other embodiments. In certain embodiments, thedeviation is about ±2 degrees. At each CNT layer 430, the CNTs 450 arearranged with a density in a range from about 50 tubes/μm to about 300tubes/μm in some embodiments, and in other embodiments, the density isin a range from about 100 tubes/μm to about 200 tubes/μm. The length ofthe CNTs 450, here in the x-axis, is in a range from about 0.5 μm toabout 5 μm in some embodiments, and is in a range from about 1 μm toabout 2 μm in other embodiments. The average diameter of the CNTs 450 isin a range from about 1.0 nm to about 2.0 nm in some embodiments.

Carbon nanotubes can be formed by various methods, such as arc-dischargeor laser ablation methods. The formed CNTs are dispersed in a solvent,such as sodium dodecyl sulfate (SDS). The CNTs can also be formedthrough chemical vapor deposition (CVD) on a quartz or sapphiresubstrate. The formed CNTs can then be transferred to and disposed onthe substrate 400 to become CNTs 450, using various methods, such as afloating evaporative self-assembly method in some embodiments.

After the CNTs 450 of the lowest CNT layer 430 are disposed on theinsulation layer 420, a buffer layer 440 is formed over the CNTs 450 ofthe lowest CNT layer 430. In some embodiments, the buffer layer 440includes a polycrystalline or amorphous semiconductor material of one ofSi, Ge and SiGe. In other embodiments, the buffer layer 440 includes adielectric material similar to or different from that of the insulationlayer 420. For example, the buffer layer 440 may be one or more layersof silicon oxide, silicon nitride, SiON, SiOC, SiOCN or SiCN, or othersuitable dielectric material. In some other embodiments, the bufferlayer 440 may include organic materials, such as organic polymers. Thebuffer layer 440 can be formed by suitable film formation methods, suchas CVD, PVD or ALD. In one embodiment, ALD is used to form the bufferlayer 440 for its high thickness uniformity and thicknesscontrollability.

In some embodiment, the formation of the buffer layer 440 may include atwo- step process. When the first layer of the buffer layer 440 isconformally formed over the CNT layer 430, the upper surface of thefirst layer of the buffer layer 440 includes a wavy shape having peaksand valleys. The thickness of the first layer of the buffer layer 440 isin a range from about 2 nm to about 10 nm in some embodiments, and is ina range from about 3 nm to 5 nm in other embodiments.

In some embodiments, after the first layer of the buffer layer 440 hasbeen formed with the wavy upper surface, one or more planarizationprocesses are performed to flatten the upper surface of the first layerof the buffer layer 440. The planarization operation includes anetch-back process or a chemical mechanical polishing (CMP) process. Inone embodiment, a CMP operation is performed.

Then, a second layer of the buffer layer 440 is formed over the firstlayer. In some embodiments, the second layer of the buffer layer 440includes the same material as the first layer in some embodiments. Thethickness of the second layer of the buffer layer 440 is substantiallythe same as the thickness of the first layer. The difference in thethickness is within ±5% in some embodiments with respect to the averagethickness.

After the buffer layer 430 has been formed over the lowest CNT layer430, a second CNT layer 430 of CNTs 450 is disposed on the buffer layer440. When the upper surface of the first layer of the buffer layer 430has the wavy shape, the second CNT layer 430 of CNTs 450 may tend to bearranged at the valleys of the wavy shape.

The forming a CNT layer 430 of CNTs 450 and forming a buffer layer 440over the CNT layer 430 are repeated to form n buffer layers 440 thateach encapsulate a CNT layer 430 therebelow, where n is an integer ofthree or more. In some embodiments, n is up to 20. As shown in FIG. 4, nis 7 and the wafer 400 include 7 CNT layers 430 of CNTs 450. Each CNTlayer 430 is embedded in or encapsulated by a respective buffer layer440 thereover. Each CNT layer 430 is positioned over a buffer layer 440,except for the lowest CNT layer 430 that is positioned over theinsulation layer 420.

In other embodiments, after the first buffer layer 440 is formed withthe wavy upper surface, one or more planarization operations areperformed to flatten the upper surface of the buffer layer 440. Theplanarization operation includes an etch-back process or a chemicalmechanical polishing (CMP) process. In one embodiment, CMP is used.

In an embodiment, the CNTs 450 in a same CNT layer 430 are arranged in asubstantially constant pitch and the CNTs 450 in the vertical directionare substantially aligned. However, the arrangement of the CNTs 450 inthe buffer layer 440 may also have random pitch within a CNT layer 430,e.g., in the y-axis orientation. In some embodiments, when the averagediameter of the CNTs 450 is D_(CNT), the horizontal pitch P_(H) of theCNTs 450 is D_(CNT)≤P_(H)≤10×D_(CNT). In some embodiments, two laterallyadjacent CNTs 450, e.g., in the same CNT layer 430, may be in contactwith one another. Further, in the vertical direction, e.g., z-axis, someCNTs 450 in different CNT layers 430 may not be aligned with oneanother, in some embodiments. The vertical pitch Pv of the CNTs 450 isdetermined by the thickness of the buffer layers 440. In someembodiments, a vertical pitch Pv of the CNTs 450 between immediatelyadjacent CNT layers 430 is 0.9×P_(Average)≤P_(V)≤1.1×P_(Average), whereP_(A) is an average pitch of the multiple CNT layers 430, e.g., inz-axis. In other embodiments, the vertical pitch P_(V) is0.95×P_(A)≤P_(V)≤1.05×P_(A).

In some embodiments, after the CNTs 450 are transferred over thesubstrate 410, a trimming process is performed through etching.

In example operation 310, with reference also to FIG. 5, fin structures512, 514, 516, 518 are formed by patterning the CNT layers 430 and thebuffer layers 440. The patterning may use one or more lithography andetching operations with a mask pattern formed over the top level bufferlayer 440. The buffer layers 440 and the CNT layers 450 are patternedinto one or more fin structures 510, shown as four fin structures 512,514, 516, 518. In the description herein, the fin structures aregenerally referred to as fin structures 510 unless a specific finstructure 512, 514, 516, 518 is referred to in applicable scenarios. Themask pattern may be a photo resist layer in some embodiments, or a hardmask made of dielectric material in some other embodiments. In someembodiments, the fin structures 510 may be patterned by any suitablemethod. For example, the fin structures 510 may be patterned using oneor more photolithography processes of extreme ultraviolet (EUV)lithography, double-patterning or multi-patterning processes or otherphotolithography processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, which achieve patterns of smaller pitches than thoseobtainable using a single, direct photolithography process.

In some embodiments, the width 522 of the fin structures 510 in they-axis orientation is in a range from about 5 nm to about 20 nm, and isin a range from about 7 nm to about 12 nm in other embodiments. In anembodiment, as shown in FIG. 5, the insulation layer 420 is made of adifferent material than the buffer layers 440 and thus the insulationlayer 420 is not patterned. In some other embodiments, a lowest bufferlayer 440 is formed between the insulation layer 420 or the substrate410 and the lowest CNT layer 430. The lowest buffer layer 440 is alsopatterned into the fin structures 510.

The total number of the CNTs 450 contained in a fin structure 510 is ina range from about 5 to about 100 in some embodiments, and is in a rangefrom about 10 about 50 in other embodiments. In each CNT layer 430 overa buffer layer 440 contained in a fin structure 510, the number of CNTs450 may vary in a range between 1 to 15 to 15 CNTs 450 depending on thedevice designs and/or configurations. In an embodiment, each CNT layer430 in a fin structure 510 includes 3 CNTs 450.

Various configurations of CNTs 450 are possible in a fin structure 512,514, 516, 518. For example, some CNTs 450 may be partially exposed at aside surface of the buffer layer 440 in some embodiments. In such acase, a removal operation may be performed to remove the partiallyexposed CNTs 450. The removal operation can be a plasma treatment usingoxygen containing gas.

In some embodiments, the number of CNTs 450 contained in a CNT layer 430may vary among CNT layers 430 in a same or different fin structure 510.Further, the pitch of the CNTs 450 in one CNT layer 430 may be differentfrom the pitch of the CNTs 450 in another CNT layer 430 in someembodiments. The pitch of the CNTs 450 may vary within a same CNT layer430 in some embodiments. Adjacent CNTs 450 in one CNT layer 430 may bein contact with one another in some embodiments, or may be discrete fromone another in some other embodiments. The CNTs 450 in different CNTlayer 430 of a same fin structure 510 do not contact with one another insome embodiments.

In example operation 315, with reference also to FIGS. 6 and 7,sacrificial gate structures are formed over the fin structures 510. Asshown in FIG. 6, a sacrificial gate electrode layer 612 and a gatedielectric layer 614 are blankly deposited over the fin structures 510such that the fin structures 510 are fully embedded in the sacrificialgate electrode layer 612. The sacrificial gate electrode layer 612includes silicon, germanium or silicon germanium or other suitablematerials. For example, the sacrificial gate electrode layer 612 ispolycrystalline silicon or amorphous silicon. The thickness of thesacrificial gate electrode layer 612 is in a range from about 80 nm toabout 200 nm in some embodiments. In some embodiments, the sacrificialgate electrode layer 612 is subjected to a planarization operation. Thesacrificial gate electrode layer 612 is deposited using CVD, PVD, ALD,or other suitable processes. The gate dielectric layer 614 is optionaland in some embodiments, no sacrificial gate dielectric layer is formedbelow the sacrificial gate electrode layer 612 depending on process ordevice designs.

A mask layer 620 is formed over the sacrificial gate electrode layer612. The mask layer 620 includes one or more of a silicon nitride layer,a silicon oxide layer or other suitable hard mask layers.

Referring to FIG. 7, with the mask layer 620 patterned, the sacrificialgate electrode layer 612 is patterned into sacrificial gate electrodes712, shown as four sacrificial electrodes 712 for illustrative purposes.The sacrificial gate electrode 712 and the gate dielectric layer 614form sacrificial gate structure 710. With the sacrificial gateelectrodes 712 being formed, the fin structures 510 are each partiallyexposed on opposite sides of the sacrificial gate electrodes 712. Theportions of the fin structures 510 that are covered by the sacrificialgate electrodes 712 are referred to as “channel portions” 722 and theportions of the fin structures 510 that are exposed from the sacrificialgate electrodes 712 are referred to as “extension portions” 724, fordescriptive purposes. In an embodiment, the source/drain (S/D) regionsof a device are generally defined by the extension portions 724 of thefin structures 512. In an embodiment, a source and a drain of a deviceare interchangeably used and the structures thereof are substantiallythe same. FIG. 7 shows that four sacrificial gate electrodes 712 areformed over four fin structures 510, but the number of the sacrificialgate structures is not limited to this configuration. One or more thanone sacrificial gate structures can be arranged in the x-axis directionin some embodiments. In certain embodiments, one or more sacrificialgate electrodes 712 are configured as dummy gate structures to improvepattern fidelity and/or structural integrity. A dummy gate structurerefers to a gate structure that is not configured to control the flow ofcharge carriers. A dummy gate structure may include a same structuralconfiguration as a normal gate structure except for the functionalconfiguration thereof

Optionally, an outer spacer structure is formed adjacent to thesacrificial gate electrodes 712. The outer spacer structure (not shownfor simplicity) is conformally formed adjacent to the sacrificial gateelectrodes 712 using CVD or other suitable methods. The layer of theouter spacer is deposited in a conformal manner so that it is formed tohave substantially equal thicknesses on vertical surfaces, such as thesidewalls, horizontal surfaces, and the top of the sacrificial gateelectrodes 712. In some embodiments, the outer spacer layer has athickness ranging from about 2 nm to about 10 nm. In some embodiments,the dielectric material of the outer spacer structure is one or more ofSiN, SiON, SiOCN, SiCN, or SiOC or some other suitable dielectricmaterials. The outer spacer layer is etched through anisotropic etching,e.g., reactive ion etching (RIE), to form the outer spacers. During theanisotropic etching process, most of the dielectric material of theouter spacer layer is removed from the horizontal surfaces, e.g., thex-y plane, leaving the dielectric spacer layer on the vertical surfaces,such as the sidewalls of the sacrificial gate electrodes 712 and thesidewalls of the extension portions 724 of the fin structures 510. Insome embodiments, an isotropic etching process may be subsequentlyperformed to remove the dielectric material of the outer spacer layerfrom the extension portions 724 of the fin structures 510.

Optionally, a liner layer, such as an etch stop layer, is formed tocover the sacrificial gate electrodes 712, the outer spacer structuresand the extension portions 724 of the fin structures 510. In someembodiments, the liner layer includes a silicon nitride-based material,such as silicon nitride, SiON, SiOCN or SiCN and combinations thereof,formed by CVD (including LPCVD and PECVD), PVD, ALD, or other suitableprocess.

In example operation 320, with reference also to FIG. 8, a firstinterlayer dielectric (ILD) layer 810 is formed (FIG. 8). The materialsfor the first ILD layer 810 include compounds comprising Si, O, C and/orH, such as silicon oxide, SiCOH and SiOC. Organic materials, such aspolymers, may also be used for the first ILD layer 810.

In example operation 325, with reference to FIG. 9, the sacrificialgates 712 are exposed from the ILD layer 810. In an embodiment, as shownin FIG. 9, the sacrificial gates 712 are exposed by selectively removingthe mask layer 620. In another embodiment, a planarization operation,such as CMP, is performed to remove the mask layer 620 and upper portionof the ILD layer 810 so that the sacrificial gate electrodes 712 areexposed.

In example operation 330, with reference also to FIG. 10, thesacrificial gate electrodes 712 are removed to form gate spaces 1010,thereby exposing the channel portions 722 of the fin structures 510 bythe gate spaces 1010. The sacrificial gate electrodes 712 can be removedusing plasma dry etching and/or wet etching. For example, in the casethat the sacrificial gate electrodes 712 is polysilicon and the ILDlayer 810 is silicon oxide, a wet etchant such as a TMAH solution can beused to selectively remove the sacrificial gate electrode 712.

The gate dielectric layer 614 may be removed subsequent to the removingthe sacrificial gate electrodes 712 or may remain after the removing thesacrificial gate electrodes 712.

FIG. 10 shows, as an illustration, that all the sacrificial gateelectrodes 712 are removed. The disclosure is not limited by thisexample. Some of the sacrificial gate electrodes 712 may remain tofunction as dummy gate structures or may be removed in some subsequentoperations to form, e.g., a diffusion break structure.

In example operation 335, with reference also to FIG. 11, the CNTs 450are released from the channel portion 722 of the fin structures 510.Specifically, the buffer layers 440 in the channel portions 722 of thefin structures 510 are removed to release the CNTs 450. The bufferlayers 440 can be removed selectively to the CNTs 450 and the ILD layer810 using plasma dry etching and/or wet etching. When the buffer layers440 are polysilicon or amorphous silicon and the first ILD layer 810 issilicon oxide, a wet etchant such as a TMAH solution is used. In anembodiment, in a case that the sacrificial gate electrode 712 and thebuffer layers 440 are made of the same material, the removal of thesacrificial gate electrodes 712 and the removal of the buffer layers 440in the channel portion 722 may be achieved by a same etching operation.

The portions of the CNTs 450 that are released from the channel portion722 are referred to as channel portions 1110 of the CNTs 450. In someembodiment, some portions 1112 of the CNTs 450 under the ILD layer 810are also released due to the undercut region 1114 formed by the etchingoperation. The portions 1112 of the CNTs 450 may be configured to besource/drain extension portions 1112 and may be doped to enhance theelectrical characteristics of the devices. In some other embodiment, forexample, when the silicon or silicon germanium nanowires or nanosheetsare used as the semiconductor strips, the source/drain extension portionmay not be formed.

In some embodiment, the undercut regions 1114 may also be used to forman inner spacer structure (not shown for simplicity) therein. The innerspacer structure may be configured to provide insulation between thegate structure and the source/drain structures.

In example operation 340, with reference to FIGS. 12-15, after thechannel portions 1110 of the CNTs 450 are released, replacement gatestructures 1510 are formed adjacent to, e.g., wrapping around, thechannel portions 1110 of the CNTs 450. In some embodiment, in a casethat the source/drain extension portions 1112 are also released, thereplacement gate structures 1510 are also formed adjacent to thesource/drain extension portions 1112.

Specifically, as shown in FIG. 12, a gate dielectric layer 1210 isformed around the channel portions 1110 of the CNTs 450. In someembodiments, the gate dielectric layer 1210 includes one or more layersof a dielectric material, such as silicon oxide, silicon nitride, or ahigh-K dielectric material, or other suitable dielectric material,and/or combinations thereof. Examples of high-K dielectric materialinclude HfO₂, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, zirconium oxide,aluminum oxide, titanium oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃)alloy, or other suitable high-K dielectric materials, and/orcombinations thereof In some embodiments, the gate dielectric layer 1210is made of HfO₂ for an nFET device, and is made of Al₂O₃ for a pFETdevice. The gate dielectric layer 1210 has a thickness in a range fromabout 0.5 nm to about 2.5 nm in some embodiments, and has a thickness ina range from about 1.0 nm to about 2.0 nm in other embodiments. The gatedielectric layer 1210 may be formed by CVD, ALD or other suitablemethod. In one embodiment, the gate dielectric layer 1210 is formedusing a highly conformal deposition process such as ALD in order toensure a uniform thickness around each channel portions 1110 of the CNTs450.

In some embodiments, an interfacial layer (not shown) is formed aroundthe channel portions 1110 of the CNTs 450 before the gate dielectriclayer 1210 is formed. The interfacial layer is made of a dielectricmaterial, e.g., SiO₂, and has a thickness in a range from about 0.5 nmto about 1.5 nm in some embodiments. In other embodiments, the thicknessof the interfacial layer is in a range from about 0.6 nm to about 1.0nm.

As shown in FIG. 13, in certain embodiments, one or more work functionadjustment layers 1310 are formed over the gate dielectric layer 1210.The work function adjustment layers 1310 are made of a conductivematerial such as a single layer of TiN, TaN, TaAlC, TiC, TaC, Co, Al,TiAl, HfTi, TiSi, TaSi or TiAlC, or a multilayer of two or more of thesematerials. In certain embodiments, TiN is used as the work functionadjustment layer 1310. The work function adjustment layer 1310 may beformed by ALD, PVD, CVD, e-beam evaporation, or other suitable process.Further, the work function adjustment layer 1310 may be formedseparately for nFET or the pFET using different metal materials.

As shown in FIG. 14, a gate electrode layer 1410 is formed over the workfunction adjustment layer 1310. The gate electrode layer 1410 includesone or more layers of conductive material, such as polysilicon,aluminum, copper, titanium, tantalum, tungsten, cobalt, molybdenum,tantalum nitride, nickel silicide, cobalt silicide, TiN, WN, TiAl,TiAlN, TaCN, TaC, TaSiN, metal alloys, other suitable materials, and/orcombinations thereof. The gate electrode layer 1410 has a thickness in arange from about 0.5 nm to about 5.0 nm in some embodiments, and has athickness in a range from about 0.8 nm to about 1.5 nm in otherembodiments. The gate electrode layer 1410 may be formed by CVD, ALD,electro-plating, or other suitable method.

As shown in FIG. 15, the replacement gate structures 1510 are formed byremoving the excess materials of the gate electrode layer 1410, the gatedielectric layer 1210, the work function adjustment layer 1310 over theupper surface 1520 of the ILD layer 810 through a planarizationoperation, e.g., a CMP operation, at least until the ILD layer 810 isrevealed. In some embodiment, an upper portion of the ILD layer 810 mayalso be removed by the planarization operation.

The replacement gate structure 1510 includes the gate dielectric layer1210, the work function adjustment layer 1310 and the gate electrodelayer 1410. One or more of the gate dielectric layer 1210, the workfunction adjustment layer 1310 and the gate electrode layer 1410 wraparound the channel portions 1110 of the CNTs 450. The extension portions1112 of the CNTs 450 may be doped to function as source/drain extensionportions or may be maintained as intrinsic to become part of thechannels 1110.

In some embodiment, the gate dielectric layer 1210 fully wraps aroundthe channel portions 1110 of the CNTs 450. And the work functionadjustment layer 1310 also fully wraps around the channel portions 1110of the CNTs 450. In some embodiments, spaces are formed between the workfunction adjustment layer 1310 and the spaces are filled by the gateelectrode layer 1410. The gate electrode layer 1410 may not wrap aroundthe channel portions 1110 of the CNTs 450.

FIG. 15 shows, as an illustrative example, that the gate structure 1510does not fully wrap up the channel portion 1110 of the lowest CNT 450that is positioned on the insulation layer 420. This example does notlimit the scope of the disclosure. In some embodiment, the channelportion 1110 of the lowest CNT 450 may be fully released by removing theupper portion of the insulation layer 420.

It is possible that some of the sacrificial gate structures 710 may notbe replaced by replacement gate structures 1510 and may remain after thereplacement gate structures 1510 are formed.

In example operation 345, with reference to FIGS. 16-18, diffusion breakstructures 1810 are formed between gate structures 1510 or 710. Thediffusion break structure 1810 is a dielectric body that laterallyseparates the CNTs 450. Therefore, the diffusion break structures 1810borders a separated active region or circuit region 1820. Chargecarriers do not flow in or out of the separated active region 1810through the CNTs 450 because the CNTs 450 are truncated by the diffusionbreak structure 1810.

Specifically, FIG. 16 shows an example double diffusion breakembodiment. A trench 1610 is formed adjacent to a gate structure 1510 or710. The trench 1610 is formed via etching with a mask layer 1620 formedand patterned over the wafer 400. An opening 1622 of the patterned masklayer 1620 overlaps the extension portions 724 of the CNTs 450.Selective wet etching or dry etching operations are used to remove theILD layer 810 and the buffer layers 440 either sequentially or together,depending on the materials of the buffer layers 440 and the ILD layer810. After the ILD layer 810 and the buffer layers 440 are removed inthe trench 1610, the extension portions 724 of the CNTs 450 aretruncated in the trench 1610 by etching. It should be appreciated thatthe extension portions 724 of the CNTs 450 may not be fully removed fromthe trench 1610. The etching may truncate the extension portions 724within the trench 1610 while leaving some residual CNTs 450 remaining inthe trench 1610, as illustratively shown in FIG. 16D, which is a topview of the trench 1610. As the trench 1610 is formed by terminating anextension portion 724 of the CNTs 450, the trench 1610 is more adjacentto a channel portion 722 of the CNT 450 than another extension portion724 of the CNT 450.

FIG. 17 shows a B-plane view of an alternative embodiment where a singlediffusion break 1710 is formed by removing at least partially a gatestructure 1510, 710. As mentioned herein, some sacrificial gate 710 maynot be replaced with the replacement gate structure 1510 and may be keptfor structural integrity, e.g., as dummy gates, or for forming thesingle diffusion break structure 1710. FIG. 17 shows a trench 1710 thatis formed by opening a replacement gate structure 1510, as anillustrative example. It should be appreciated that similar descriptionalso applies to opening a sacrificial gate structure 710 in forming thetrench 1710.

As shown in FIG. 17, an opening 1720 of the mask layer 1620 overlaps achannel region 722 of the CNTs 450. Selective wet etching or dry etchingoperations may be performed to remove the gate electrode 1410 and thework function adjustment layer 1310, either sequentially or together,depending on the choices of materials of the gate electrode 1410 and thework function adjustment layer 1310. After the gate electrode 1410 andthe work function adjustment layer 1310 are removed, the channelportions 722 of the CNTs 450 are truncated in the trench 1710 by, e.g.,anisotropic etching or selective etching.

The trenches 1610, 1710 may be sufficiently deep to truncate therespective portions of the CNTs 450, while the trenches 1610, 1710 donot extend into the insulation layer 420 or the substrate 410 and maystop at a point about the upper surface of the insulation layer 420 orthe substrate 410. For example, in a case that the lowest CNTs 450 arepositioned over a semiconductor buffer layer 440 that is depositeddirectly on the substrate 410, the trenches 1610, 1710 do not extendinto the substrate 410. In an embodiment, the trenches 1610, 1710 mayextend into layers under the lowest CNTs 450 due to unintentionalprocess control variations, which does not deviate from the principlesof the disclosure.

Then, as shown in FIG. 18, the trench 1610, 1710 is filled with adielectric body 1810. The dielectric body 1810 is silicon oxide, siliconnitride, a low-K dielectric material or other suitable dielectricmaterial. FIG. 18 follows from the double diffusion break embodiment ofFIG. 16 for illustrative purposes, which does not limit the scope of thedisclosure. Similar descriptions also apply to a dielectric body filledwithin the trench 1710 of single diffusion break. The dielectric body1810 functions as a diffusion break structure 1810 to separate theactive region or circuit region 1820 from the rest portions 1830 of asame IC device. Specifically, charge carriers are blocked by thediffusion break structure 1810 from flowing between the active region1820 and the rest portion 1830 through the CNTs 450.

As shown in FIG. 18, the diffusion break structure 1810 separates andblocks all the CNTs 450 of the active region 1820. In some otherembodiments, not all the CNTs 450 in the active region 1820 are used aschannel strips for charge carrier flow. For example, the channelreleasing operation of FIG. 11 may only release some of the upper CNTs450, while leaving the lower CNTs 450 remain encapsulated by the bufferlayers 440. The lower CNTs 450 are thus not configured as channelstrips. In that scenario, the diffusion break structure 1810 may have adepth to truncate or block only those upper CNTs 450 that are used aschannel strips for the respective devices.

After the diffusion break structure 1810 are formed, the CNTs 450 areseparated into a first array of CNTs 450 by one side of the diffusionbreak structure 180 and a second array of CNTs 450 by the opposing sideof the diffusion break structure 180. The CNTs 450 of the two arraysseparated by the diffusion break structure 1810 are in lateral alignmentwith one another because the two arrays belong to the same verticalstack of CNTs 450 before the diffusion break structure 1810 is formed.

In example operation 350, with reference also to FIGS. 19-23,source/drain structures 2010, 2110, 2210 are formed in the active area1820 adjacent to the replacement gate structures 1510. As shown in FIG.19, the active area 1820 is configured to have an nFET area 1932 and apFET area 1922. As an illustrative example, the nFET area 1932 includesCNTs 450 in the fin structures 512, 514. The pFET area 1922 includesCNTs 450 in the fin structures 516, 518. The extension portions 724 ofthe CNTs 450 are released in source/drain openings 1910 in the nFET area1932. With a mask 1920 covering the pFET area 1922 and a photoresistlayer 1930 patterned over the nFET area 1932, source/drain openings 1910are formed by removing the ILD layer 810 and by releasing CNTs 450 inthe extension portions 724 of the fin structures 510. After the bufferlayers 440 in the extension portions 724 are removed, the release CNTs450 become the source/drain portions 1950 of the CNTs 450. Thesource/drain portions 1950 are connected to the channel portions 1110through source/drain extension portions 1012 of the CNTs 450. In someembodiment, the source/drain extension portions 1012 are configured aspart of the channel portion 1010.

Optionally, after source/drain openings 1910 are formed, an inner spacerstructure is formed within the source/drain openings 1910 and adjacentto the gate structure 1510. The inner spacer structure includes one ormore layers of dielectric materials.

Next, as shown in FIG. 20, source/drain structures 2010 are formedwithin the source/drain openings 1910 by filling the source/drainopenings 1910 with one or more layers of a conductive material. Theconductive material includes one or more of W, Cu, Co, Ti, Ag, Al, TiAl,TiAlN, TaC, TaCN, TaSiN, Mn, Co, Pd, Ni, Re, Ir, Ru, Pt, and Zr, oSc,Er, Y, La, or any other suitable conductive materials. In someembodiments, the source/drain structure 2010 include a first layer orlower contact layer 2012 and a second layer or an upper contact layer2014. The lower contact layer 2012 wraps around the source/drainportions 1950 of CNTs 450 and an upper contact layer 2014 is formed overthe lower contact layer 2012. In some embodiments, the lower contactlayer 2012 is configured as a work function adjustment layer. The lowercontact layer 2012 is Pd, Pt, Ru, Ni, Mg, for pFET or Sc, Er, Y, La, Ni,Mg for nFET. The upper contact layer 2014 is one or more of W, Cu and Coin some embodiments, which are metal materials suitable to beinterconnection structures under the back-end-of-line processes. Theupper contact layer 2014 may be configured as a source/drain electrode.In some further embodiment, a third contact layer is formed between theCNTs 450 and the lower contact layer 2012.

In an embodiment, as shown in FIG. 20, the upper contact layer 2014 doesnot extend downward vertically between the source/drain portions 1950 ofthe CNTs 450. The disclosure is not limited by this example. In otherexamples, the upper contact layer 2014 may extend downward besidesand/or between the source/drain portions 1950 of the CNTs 450.

In the example of FIG. 20, the source/drain structure 2010 wraps aroundthe source/drain portions 1950 of the CNTs 450. That is, thesource/drain structures 2010 contacts the CNTs 450 by the sidewallsurface thereof.

In another embodiment, as shown in FIG. 21, the source/drain portions1950 of the CNTs 450 are at least partially receded and the source/drainstructures 2110 contact edge surfaces 2120 of the receded CNTs 450. Theedge surfaces 2120 may be formed as part of the remaining source/drainportions 1950 or may be part of the source/drain extension portions 1012or the channel portions 1110 of the CNTs 450. Similar to thesource/drain structure 2010, the source/drain structure 2110 may alsoinclude a lower contact layer 2112 and an upper contact layer 2114.

As shown in FIG. 22, source/drain structures 2210 are formed in the pFETarea 1922. The source/drain structure 2210 for the pFET may include adifferent material for the lower contact layer 2212 from that of thelower contact layer 2012, 2112 for the nFET. The source/drain structure2210 for the pFET may include a same upper contact layer 2214 as that ofthe upper contact layer 2014, 2112 for the nFET.

FIG. 23 shows a structure 2300 in a perspective view. Referring to FIGS.20-23 together, the structure 2300 includes an active area or circuitarea 1820 that is bordered by a diffusion break structure 1810. Theactive area 1820 includes an nFET area 1932 and a pFET area 1922 whichboth have semiconductor strips 450 configured for charge carrier flows.The diffusion break structure 1810 has a depth that is sufficient totruncate all the CNTs 450 that are configured for charge carrier flowsfor the nFET area 1932 or the pFET area 1922. In an embodiment, thediffusion break structure 1810 does not extend into a layer that ispositioned immediately below the lowest CNT 450. In each of the nFETarea 1932 or the pFET area 1922, gate structures 1510 wrap around atleast some of the channel portions 1110 of the CNTs 450. Thesource/drain structures 2010, 2110, 2210 are positioned adjacent to therespective gate structures 1510 and contact the CNTs 450 by one or moreof the sidewall surfaces or the edge surfaces of the CNTs 450.

In the example of FIG. 23, an nFET device in the nFET area 1932 and apFET device in the pFET area 1922 are arranged laterally with respect toone another. The disclosure is not limited to this example. In someother embodiment, the nFET and the pFET are arranged vertically withrespect to one another, e.g., in complementary FET devices.Specifically, a first set of semiconductor strips of the nFET and asecond set of semiconductor strips of the pFET may be stacked verticallywith respect to one another. The first set of semiconductor strips andthe second set of semiconductor strips may be adjacent to a common gatestructure. The diffusion break structure 1810 may be configured totruncate the first set of semiconductor strips, the second set ofsemiconductor strip or both the first set and the second set ofsemiconductor strips.

The description herein uses CNT as illustrative example of semiconductorstrips, which does not limit the scope of the disclosure. Similardescriptions also apply to semiconductor strips of other materials orstructural configurations. For example, the semiconductor strips may benanowire or nanosheet strips of silicon, silicon germanium or galliumnitride.

The advantages and features of the disclosure are further appreciablethrough the following example embodiments:

In an embodiment, a method forms a gate-all-around field effecttransistor. A vertical stack of layers are formed. The vertical stack oflayers includes semiconductor strips and buffer layers stacked in analternating manner over a substrate. A first sacrificial gate structureis formed over the vertical stack of layers. A dielectric layer isformed over the first sacrificial gate structure and the vertical stackof layers. A first channel portion of the vertical stack of layers isexposed by removing the first sacrificial gate structure. A first subsetof the semiconductor strips are released in the first channel portion byremoving at least part of the buffer layers from the first channelportion. A first replacement gate structure is formed adjacent to thereleased first subset of the semiconductor strips. A dielectric body isformed adjacent to the first replacement gate structure. The dielectricbody truncates each of the first subset of the semiconductor strips. Asource/drain structure is formed adjacent to the first replacement gatestructure.

In another embodiment, a structure includes a substrate, a firstvertical array of semiconductor strips that are separated from oneanother and over the substrate, a first gate structure adjacent to eachof the first vertical array of semiconductor strips, a second verticalarray of semiconductor strips that are separated from one another andover the substrate, a second gate structure adjacent to each of thesecond vertical array of semiconductor strips, and a dielectric bodyover the substrate and laterally between the first vertical array ofsemiconductor strips and the second vertical array of semiconductorstrips.

In a further embodiment, a semiconductor structure includes a substrate,a first vertical array of semiconductor strips that are separated fromone another, a first gate structure adjacent to each of the firstvertical array of semiconductor strips, and a first source/drainstructure adjacent to the first gate structure and contacting the firstvertical array of semiconductor strips. The semiconductor structure alsoincludes a second vertical array of semiconductor strips that areseparated from one another, a second gate structure adjacent to each ofthe second vertical array of semiconductor strips, and a secondsource/drain structure adjacent to the second gate structure andcontacting the second vertical array of semiconductor strips. Thesemiconductor structure also includes a dielectric body over thesubstrate and laterally between the first source/drain structure and thesecond source/drain structure. The dielectric body is more adjacent tothe first source/drain structure than to any gate structure in thesemiconductor structure.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method, comprising: forming a vertical stack oflayers including carbon nanotube layers and buffer layers stacked in analternating manner over a substrate; forming a first sacrificial gatestructure over the vertical stack of layers; exposing a first channelportion of the vertical stack of layers by removing the firstsacrificial gate structure; releasing a first subset of the carbonnanotube layers in the first channel portion by removing at least partof the buffer layers from the first channel portion; forming a firstreplacement gate structure adjacent to the released first subset of thecarbon nanotube layers; and forming a dielectric body adjacent to thefirst replacement gate structure, the dielectric body truncating each ofthe first subset of the carbon nanotube layers, the forming thedielectric body including: removing a gate electrode layer in a secondgate structure adjacent to the first replacement gate structure toexpose the first subset of carbon nanotube layers under the second gatestructure; and removing portions of the first subset of carbon nanotubelayers exposed under the second gate structure.
 2. The method of claim1, wherein the dielectric body extends downward and ends at a pointhigher than an upper surface of the substrate.
 3. The method of claim 1,further comprising: forming an insulation layer between the substrateand the vertical stack of layers including the carbon nanotube layersand the buffer layers.
 4. The method of claim 1, wherein the substrateis a semiconductor material and the vertical stack of layers includes abuffer layer at a bottom of the vertical stack of layers and in contactwith the substrate.
 5. The method of claim 1, wherein the buffer layersinclude a polycrystalline or amorphous material of one of Si, Ge orSiGe.
 6. The method of claim 1, wherein the buffer layers include adielectric material.
 7. The method of claim 1, further includingremoving a gate dielectric layer of the second gate structure before theremoving the portions of the first subset of carbon nanotube layersexposed under the second gate structure.
 8. The method of claim 1,wherein the removing the gate electrode layer in the second gatestructure includes removing a gate electrode layer of a replacement gatestructure.
 9. The method of claim 1, wherein the removing the gateelectrode layer in the second gate structure includes removing a gateelectrode layer of a sacrificial gate structure.
 10. The method of claim1, wherein the dielectric body separates all the carbon nanotube layersof the vertical stack of layers and extends to an upper surface of thesubstrate.
 11. A method comprising: forming a vertical stack of layersincluding semiconductor strips and buffer layers stacked in analternating manner over a substrate; exposing a first channel portion ofthe vertical stack of layers; releasing a first subset of thesemiconductor strips in the first channel portion by removing at leastpart of the buffer layers from the first channel portion; forming afirst replacement gate structure wrapping around the released firstsubset of the semiconductor strips; forming a dielectric body adjacentto the first replacement gate structure, the dielectric body truncatingeach of the first subset of the semiconductor strips, the forming thedielectric body including: removing a gate structure adjacent to thefirst replacement gate structure to expose the first subset ofsemiconductor strips under the gate structure; and removing portions ofthe first subset of semiconductor strips exposed under the gatestructure.
 12. The method of claim 11, wherein the gate structure is asacrificial gate structure.
 13. The method of claim 11, wherein the gatestructure is a replacement gate structure.
 14. The method of claim 11,further comprising: forming a source/drain structure that wraps aroundand contacts a longitudinal sidewall of one or more of the semiconductorstrips under the first gate structure.
 15. The method of claim 11,further comprising: forming a source/drain structure that laterallycontacts an edge surface of one or more of the semiconductor strips ofthe vertical stack.
 16. A method, comprising: forming a dummy gatestructure wrapping around a third vertical array of semiconductor stripsbetween a first vertical array of semiconductor strips and a secondvertical array of semiconductor strips; forming a first conductive gatestructure wrapping around the first vertical array of semiconductorstrips; forming a second conductive gate structure wrapping around thesecond vertical array of semiconductor strips; and forming a dielectricbody by removing the third vertical array of semiconductor strips andreplacing the dummy gate structure with a dielectric layer.
 17. Themethod of claim 16, wherein the dielectric body laterally contacts edgesurfaces of the first vertical array of semiconductor strips andlaterally contacts edges surfaces of the second vertical array ofsemiconductor strips.
 18. The method of claim 16, wherein the formingthe dielectric body includes: forming a mask layer having an openingoverlapping channel portions of the third vertical array ofsemiconductor strips; forming a trench by etching the dummy gatestructure and the channel portions through the opening to expose anupper surface of a substrate underlying the third vertical array; andfilling the trench with a dielectric layer.
 19. The method of claim 18,wherein the etching the channel portions includes truncating carbonnanotubes of the channel portions.
 20. The method of claim 19, whereinresidual carbon nanotubes remain in the trench following the etching thechannel portions.